Video transcript

You've probably heard that the
top speed of some type of aircraft is Mach 2. Or maybe you've watched some
type of science fiction movie, or some type of fighter movie,
where they say, hey we are going to travel at Mach 1.8. And you had a sense, because
they're using it in the same context that you would use
as speed, that this is some type of speed. That these numbers represent
some type of speed. And if you assume that,
you were right. And the speeds they represent
are actually a multiple, or depending on how you want
to view it, multiples or fractions of the speed of
sound in that medium. So if someone tells you that
the Mach number is 2, what that tells you is that they are
traveling two times the speed of sound, in that medium,
for those conditions. And I make those last two
qualifiers on it because the speed of sound is not
always the same. It has a different speed
depending on whether it's traveling in air, or water, or
even depending on what makes up the air. And even if it is only traveling
in a certain type of air, or a certain makeup, a
mix of gases, it'll also change depending on
the temperature. So if you're at sea level, at
roughly about 20 degrees Celsius, the speed of sound is
300, so sea level-- do a different color, maybe blue for
sea level-- so if you're at sea level, the speed
of sound is about 340 meters per second. This is at 20 degrees Celsius,
so it's a nice comfortable temperature at sea level, which
is roughly about 760 miles per hour. Now if the temperature were to
drop, so if temperature goes down, so does the
speed of sound. And likewise, if the temperature
goes up, the speed of sound goes up. So when someone tells you that
something is traveling at Mach 2, they're saying its traveling
at two times the speed of sound for
that medium. Usually they're talking
about something traveling through air. And for the speed of sound at
the temperature that they're going through. So if they're at some super-high
altitude, and normally to travel at these
types of speeds, you have to be at a fairly high altitude
where the air is less dense. You're also traveling in an
environment where the temperature is significantly
lower than 20 degrees Celsius. So you might say, hey, does
that mean if someone's traveling at Mach 2, does that
mean that they're definitely traveling at 680 meters
per second, or at 1520 miles per hour? And of course, whenever someone
gives an air speed, it's relative to the air. And the answer is no. They're probably going to be
traveling a little bit slower than that, because at the
altitude that they're traveling the speed
of sound is lower. Now there are ground speed
records where they are traveling in kind of the Mach
range where their velocities are high enough to say
it in terms of Mach. And for those types of
situations, they are traveling at multiples of 760 miles per
hour, 340 meters per second. Now you're probably saying, hey,
who is this person right here that I've cut and pasted
from Wikipedia? And this Ernst Mach. And this is who the Mach
number is named after. Ernst Mach, and he was this
Austrian physicist and philosopher. He did a ton of research
on shockwaves and soundwaves, and whatnot. So the Mach number is
named after him.